Many devices exist for measuring the amount of liquid remaining in a vessel. In the case of motor vehicle fuel tanks, for instance, fuel level sensing is conventionally accomplished using a float connected to one end of a float arm which travels an angular path as the fuel level varies. The other end of the float arm moves a wiper contact over a resistor. Electrical connections to the wiper contact and the resistor result in a variable measure of resistance as the float moves in accordance with the level of fuel in the tank. A signal resulting from the variable resistance is sent to an indicator, such as a fuel gauge, to indicate to a vehicle operator the amount of fuel present in the tank.
In the design described above, however, the float arm is subject to interference with the walls or other internal structure of the fuel tank. Also, the resolution of the device is low because a relatively large change in the level of the fuel results in only a small movement of the wiper contact over the resistor. This is particularly disadvantageous when the fuel level nears empty. Furthermore, a float arm design may not be readily adaptable in fuel tanks which use an inflatable bladder instead of conventional solid walls.
Liquid level sensors having a float which rides up and down on a vertical member have, therefore, been proposed. For example, U.S. Pat. No. 4,827,769 to Riley et al. discloses a fuel level sensor with a vertical member and a float. A thick film cermet resistive track is deposited on one side of the vertical member, and a conductor is located on the other side of the vertical member. The float is buoyed at the level of fuel in the tank, causing the resistive track to be shorted to the conductor. This completes an electrical circuit in which the resistance is proportional to the level of fuel in the tank. Similar devices are manufactured by VDO Adolf Schindling AG, and include a float that rides up and down on several vertical nickel-chromium wires.